CLASSIFICATIONS IN NATURAL HISTORY. 213 



the elementary functions of motion, sensation, digestion, and 

 reproduction. The one differentiation, which at least numer- 

 ous kinds of the Protozoa have accomplished, is shown in the 

 secretions with which they surround themselves, constructed 

 in definite, forms, but of almost infinite variety. 



Second, the next stage of differentiation is seen in each 

 of the remaining types of animals, inclusively called the 

 Mctazoa. In all of these animals (i) there is the localization 

 of the digestive functions in the interior of the body, the 

 gastro-vascular cavity, (2) a mouth leading to this cavity, and 

 (3) the location of the motory functions on the outer side of 

 the body. In the second branch of the Animal Kingdom, 

 the Ccelenterata, there is little more of specialization of the 

 digestive functions than this, i.e. (4) there are two elemen- 

 tary tissues differentiated, and in this simplest type (as in all 

 higher) the tissues are formed in the course of individual de- 

 velopment by the segmentation (40) of the primitive cell (4^), 

 the formation of numerous cells, and then a (4*:) specialization 

 of some of these cells as tissues for one function, others of 

 them for other functions. This process, which is called de- 

 velopment, may be regarded as a specialization of the gen- 

 eralized function of reproduction. In the Protozoa repro- 

 duction takes place by simple fission and gemmation. In this 

 lowest branch of the Metazoa, the Ccelenterata, the inte- 

 grality of the body is continued after the separation into 

 parts, and what constitutes the whole of the reproductive 

 function in the Protozoa here constitutes but the segmenta- 

 tion of the contents of the egg, which differentiates the two 

 layers of tissue the Ectoderm (4^), or outside layer, and the 

 inner, or Endoderm (d.e). The fundamental function of the 

 Ectoderm is motory, the primitive function of the Endoderm 

 is digestive and assimilative. In the sponges there is de- 

 veloped between these two layers the Mesoderm (4/), in 

 which a rudimentary type of skeletal parts, in the form of 

 horny fibres, or silicious or calcareous spicules, is deposited. 

 The sponge has differentiated a digestive or gastro-vascular 

 cavity, but the mouths are several and indefinite, and the 

 cells within, by their ciliary motions, perform the functions 

 of motion as well as digestion, thus not exhibiting the full 



